The present invention relates to a method for manufacturing a printed wiring board, which employs a carbon dioxide laser to form recess portions such as via holes in a copper clad laminate to produce the printed wiring board.
In recent years, with the widespread use of cellular phones, mobile tools, and notebook personal computers, there has been an increasing demand for products which are lightweight and compact and which have a high density mounting. For this reason, a printed wiring board installed into an electronic apparatus requires the formation of fine-pitch circuits.
A printed wiring board which is lightweight and compact requires producing a circuit board having a multi-layer structure and a high density. In order to ensure interlayer electrical connections through a multi-layer printed wiring board, a conventionally used method is to form a plurality of through holes passing through a substrate. On the other hand, in recent years, for the purpose of increasing the freedom of design multi-layer printed wiring boards, a commonly used method is to form via holes having a smaller diameter than that of the through holes, or other small diameter holes such as blind via holes (BVHs) and interstitial via holes (IVHs) which do not extend through an associated substrate board but extend to the surfaces of inner copper foil layers.
In order to form the small diameter holes such as the through holes and various via holes, a conventional method is to perform mechanical drilling on a printed wiring board so as to form a plurality of small diameter holes. With the use of such a drilling treatment, a plurality of printed wiring boards may be laminated one upon another so that they can be processed simultaneously by a single drilling operation. If multiple spindle drilling is employed, it is possible to more easily improve the productivity of the drilling treatment.
Furthermore, when the drilling treatment is performed to form small diameter holes, most of such holes have a diameter of 0.3 to 0.4 mm. However, with the progress of technology during recent years, a further improved drilling treatment has become possible which can drill holes having a diameter of 0.15 to 0.25 mm.
In addition, another drilling treatment has been considered which employs a carbide drill to form holes having a diameter of 0.1 to 0.05 mm. However, this drilling treatment has been found to have many problems which have to be technically solved. For example, drills used in such a drilling treatment have only a short useful life, and there are some printed wiring boards which cannot be easily cut by drilling.
On the other hand, in the present market, the speed with which electronic devices and electric apparatus have become more compact and lightweight has been high. As a result, the present situation requires a more remarkable progress in the drilling technique. Consequently, a laser processing technique has been put into practical use to form small diameter holes having a diameter of 0.1 mm or smaller.
In the case where a laser processing technique is used to form small diameter holes in a printed wiring board, initial irradiation using laser light is allowed to start from a resin substrate (first type laser abrasion), or is permitted to start from the surface of a copper foil which will later become a desired circuit (second type laser abrasion). At this time, since the surface of a copper foil has a certain brilliance and thus has the property of reflecting laser light, the second type laser abrasion is difficult.
In fact, it is almost impossible to perform the above second type laser abrasion on the surface of a copper foil having a certain brilliance. Here, the fact that a copper foil reflects laser light will cause a decrease in the initial laser light absorbing efficiency. As a result, the speed of forming the desired holes will be undesirably reduced, thus lowering the production efficiency. For this reason, in the case where the above mentioned second type laser abrasion is carried out to form the desired holes, it is necessary to first perform an etching treatment to remove an external copper layer from an area in which the holes are to be formed. In fact, this is the conformal mask method which has already been well-known.
In the above mentioned etching treatment, it is necessary to apply an etching resist. However, since it has been difficult to control the precision of the resist application, it has been also difficult to ensure high precision of the etching position in an area where holes are to be formed. As a result, there will be a positional deviation between land pad portions forming contact points of an inner copper foil circuit and via holes to be formed during the laser abrasion. In order to solve this problem, such a positional deviation has been taken into account so that the land pad portions of the inner copper foil circuit are usually designed to have a relatively large size. This, however, is extremely unfavorable for making an inner copper foil circuit having a fine structure.
Furthermore, different sorts of lasers will of course exhibit different hole formation performances during the hole formation process. Namely, although using YAG laser will not cause any problem in treating a copper foil layer, using a carbon dioxide will make it difficult to treat the copper foil, rendering it impossible to ensure a stable hole formation process. In view of this, there has been a demand for developing an improved method of manufacturing a printed wiring board, which method does not involve an etching treatment for treating copper foil, but is capable of simultaneously stabilizing both a copper foil and a resin layer, and thus treating them using a carbon dioxide laser.